Fuses have long been necessary parts of electrical circuits, providing quick protection of circuits from overloads. Many fuses have parts that are familiar, as shown in U.S. Pat. No. 4,560,971. A fuse includes a fuse body, a conductor or fuse element within the body, and two end caps or ferrules for connection into a fuseholder or other connecting portions of a circuit. Of course, these same parts may be used in fuses large and small, and also for loads large and small. However, as circuits and circuitry get smaller, fuses also need to reduce their size. See, for example, the Nano2® Fuse series from Littelfuse, Inc., intended for use on very small surface-mounted circuits, in which many parts are actually printed onto the substrate.
When working with large fuses, it is relatively simply to assemble the fuse. The fuse element may be several inches long, as is the body, and the ferrules or other termination may be ½ inch (about 1.3 cm) or larger in diameter. Placing the parts together, by hand or in fixtures, is relatively simple and is easily accomplished. However, when the entire fuse is much reduced in size, such as fuses intended for surface-mount applications, the difficulty is increased. Many of the fuses in the above-mentioned series are less than ½ inch (about 1.3 cm) in length, and may be only about ⅛ of an inch (about 0.3 cm) in both width and depth. These small sizes are necessary to conform to the scale of the circuitry in surface-mount technology.
It is challenging to assemble reliable fuses on this small scale, principally because the requirements for reliability are high, while the size of the fuse, and all its internal bonds are small. The consistency and reliability of solder bonds are very important characteristics that must be built into the fuse. X-rays or other inspection techniques may occasionally reveal fuses which have inferior bonds. However, the trend in manufacturing is not to inspect quality into the parts, but rather to design a process in which reliability is almost inherent. That is, the present manufacturing process continually strives to consistently attain 100% reliable fuse products. Of course, the structure of a fuse, and the process for making the fuse, may vary drastically from the process described here. See, e.g., the fuses and the processes described in U.S. Pat. No. 5,977,860, and in U.S. Pat. No. 6,002,322. Fuses described in these patents are made by entirely different processes,
Without being bound to any particular theory, it is believed that the problem with the present process is that assembly requires the application of consistent external force and air pressure to reliably fit and bond the parts together. This presents no problem with the first end cap, because the opposite end is still open, allowing gas to escape. When the second end cap is secured, however, it is believed that this process traps pressurized air inside the fuse body. The pressurized gas exerts a force on the end caps, tending to move them off the fuse body. To control this, both external force and air pressure are applied. The external force holds the fuse parts together while the external air pressure will balance the pressure inside the fuse and prevent solder from coming out of the fuse assembly. This part of the process is important for attaining reliable bonds and is not easy to consistently control. Any undesirable variation in the operations will lead to inferior bonds.
The present invention is directed to solving this problem by using improved fuse components and a better process.